The present invention relates to optical fiber transmission systems using return-to-zero (RZ) pulses.
A particular advantageous application of the invention lies in systems for terrestrial applications, i.e. for providing transmission over distances of less than 4000 kilometers (km).
Long-distance single-fiber transmission systems are already known that use RZ pulses, and in particular soliton type pulses, and that make use of wavelength division multiplexing.
It has recently been shown that for distances of the order of those envisaged for terrestrial applications, it is possible to achieve transmission rates that are high (of the order of 10 gibabits per second (Gbit/s) by using RZ pulses that are relatively broad, and in particular soliton pulses having a duration of 20 picoseconds (ps) to 30 ps.
In this respect, reference can be made to the following article:
D. Le Guen, F. Favre, M. L. Moulinard, M. Henry, G. Michaud, L. Macé, F. Devaux, B. Charbonnier, T. Georges “200 Gbit/s 100 km-span soliton WDM transmission over 1000 km of standard fiber with dispersion compensation and pre-chirping”, OFC'97, paper PD 17 (Dallas, Feb. 16-21, 1997).
Nevertheless, the, spectrum occupied by 20 ps to 30 ps pulses still constitutes a limit on the density of wavelength division multiplexing that can be achieved using such systems.
In addition, cross-phase modulation phenomena modify the spectra of the signals and diffuse the energy of pulses to the sides of their spectra, far away from their center frequencies. As a result, cross-phase;modulation is responsible firstly for time deformation of the signal which gives rise in particular to interference between symbols, and secondly to crosstalk between channels when the wavelength channels are too close together. Such diffusion of energy to the sides of spectra of pulses also increases with channel power and with reduced spacing between the channels.
Furthermore, self-phase modulation phenomena can also prevent the channels being spaced too close together and can be responsible for a certain amount of spectrum broadening.
For all these reasons, the systems that have been proposed until now with channel spacing of less than 100 gigahertz (GHz) and a channel data rate of 10 Gbit/s have all been low-power transmission systems (power less than 2 decibels relative to one milliwatt (dBm)) which limits range and makes it necessary to provide a large number of amplifiers.
Consequently, it will be understood that it is desirable to be in a position to control pulse spectra closely over the entire length of a line, with this being particularly important when using broad pulses since their spectrum bandwidth is narrow.
An object. of the invention is thus to propose a solution making it possible to control the spectrum width of pulses along a transmission line in a manner that is particularly simple.
It has been known for several years that it is desirable in transmission systems using RZ pulses to have dispersion-compensating means distributed all along the transmission line.
In this respect, reference can advantageously be made to the following article: “Partial soliton communication system”, Optics Communications 87, 1992-15-18.
Document WO 97 20403 describes a soliton transmission system comprising an emitter and a receiver interconnected by an optical fiber. The fiber is subdivided into alternating sections of fiber having abnormal dispersion and sections of fiber having normal dispersion, said sections presenting lengths that are equal.
In general, such dispersion-compensating means are distributed at regular intervals along the fiber of the transmission line. They are all identical and each of them provides the same dispersion-compensation.